Boresighting is an alignment process (collimation) by which the weapon's bore centerline and gunner's optical sighting system are referred to the same aiming point. The stem or adapter (or mandrel) is the portion of the boresight device that includes a weapon insertion end which during boresighting is inserted into the bore of the weapon.
The stem or adapter generally includes a centering device which comprises plurality of flexible “fingers” that are typically equiangularly spaced about the axis of the barrel on the weapon insertion. Upon insertion into the bore of the weapon, the fingers compress within the weapon's bore and exert an outward restoring spring force to locate and secure the stem or adapter in essentially the exact center of the weapon's bore. Proper centering is known to be needed for boresighting accuracy.
Traditional stems and adapters comprise materials which provide strength as well as the required flexibility for spring action of the fingers upon insertion, such as stainless steel, stainless steel alloys, or aluminum. A boresight apparatus for zeroing the weapon is mounted on the other end of the stem or adapter so that it is positioned coaxial with the axis of the stem or adapter, such as a boresighting telescope, laser, or other zeroing device. In the case of a boresighting telescope, the boresighting telescope is positioned coaxial with the axis of the shaft of the stem or adapter, and in operation a sight may be made along the axis of the shaft to a calibration target.